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Performance Evaluation of OFDM Transmission inUAV Wireless CommunicationZhiqiang Wu Hemanth Kumar Asad DavariDept of ECE Control Systems Engineering Dept of ECE

Montgomery, WV 25136 Montgomery, WV 25136 Montgomery, WV 25136West Virginia University Institute of Technology WVUIT WVUITEmail: Zhiqiang.Wu@mail.wvu.edu

Abslmct-The use of multipIe unmanned aerial vehicles(UAVs) in collaborative missions is an increasing research areafor military planners. One of the important factors to ensurethe collaboration among swarm UAVs is the reliable, highperformancevirefesscommunication links among UAVs. Becauseof its high frequency eficiency and high performance, OFDMbased multi-carrier transmission technology ha s been consideredone of the strong candidates for swarm UAV communications.However, UAV communication links are quite different fromWireless LAN and Wireless MAN environments where OFDMi s currently used. In this paper, we thoroughly analyze the BERperformance of WE E 802.lla compatible OFDM system in aUAV w i des s communicationchamel where a large Doppler shifthas been observed and where Inter-Carrier-InterferenceUCI) ismuch larger than those of Wireless LAN and Wireless MANscenarios.I . INTRODUCTION

New generation of UAVs is emerging, using innovationsand creative enterprises, to transform both military and civilianaerospace operations and airspace operations [ I ] . One of thecrucial factors to ensure the collaboration among swarm UAVsis the reliable, high performance wireless communicationlinks among UAVs. The air-to-air UAV communication systemenables the sharing of sensor and map information amongUAVs, while an air-to-ground communication system providesmission information to the ground station for mission controland display.Due to the high BER performance and high spectral effi-ciency of OFDM based multi-carrier transmission technology[ 2 ] [ 3 ] nd particularly d ue to the success of OFDM (Orthogo-nal Frequency Division Multiplexing) technology io WirelessLAN (IEEE 802.1 I g P I , IEEE 802.1 a [4], yper LAN I1 [ 6 ] )and Wireless MAN (IEEE 802.16 [71), t is of strong interestto apply off-the-shelf OFDM based multi-carrier transmissionequipme nts (e.g., E E E 802.11a, IEEE 802.16) to suppon highdata rate w ireless communications a mong UAVs.However, the wireless communication channel in UAVcommunications is largely different from other "traditional"wireless channels. Specifically, in traditional wireless comm u-nications, one end (the base station) is fixed and the relativespeed of the other end (mob ile terminal) to base station is lessthan 80 miles per hour. By contrast, in UAV communications,007803-8608-9/D5/$20.00 0 2 0 0 5 IEEE

both ends of th e communication system are flying at a highspeed and the relative speed could be as high as 500 milesper hour. The high speed of UAVs makes the wireless channelquite challenging to deal with. Furthermore, OFDM systemscurrently are employed largely in wireless LAN and wirelessMAN scenarios where mobility is normally negligible. (Re-cently, there is some em erging research in supporting relativelyhigh mobility in Wireless MAN, e.g., IEEE 802.16 society[SI).Hence, a thorough evaluation of the performance of OFDMsystem in such a high mobility environment needs to be con-ducted. Specifically, such high mobility leads to a much shortercoherence time and a larger Doppler spread in the multi-pathfading channel. As a direct result, the orthogonality amongsubcaniers is lost and inter-carrier-interference degrades theperformance.

In this paper, we derive the design criteria of applyingOFDM echnology to UAV communications and evaluate theBE R performance of IEEE 802.11a OFDM system in a typicalUAV communication channel.Section I1 briefly outlines th e OFDM system architectures;Section U1 describes the UAV communication channel and de-sign criteria of applying OFDM technology into such channels;and Section IV provides the BER performance evaluation. Ananalysis and conclusions follow.

11. SYSTEM ARCHITECTUREF O F D MFigure 1 ilIustrates the system architecture of an OFDMsystem [2][9]. In Figure 1 , the input data stream is multiplexedinto N symbol streams, each with symbol period T,. Next,each symbol stream is used to modulate parallel, synchronoussub-carriers [I] . The sub-carriers are spaced by I / N T s infrequency domain, thus they are orthogonal over the symbolduration interval (0,Ts),.e.,T.1 cas(27T.m . 4 t - 4 , ) C O S ( 2 T I n . 4 f t + 4 n ) = 0 (1)

where m # n.The transmitted OFDM symbol can be expressed as

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Fig. 1. Block Diagram of OFDM System

where the X,,, is the baseband data symbols on mth sub-carrier.Upon transmission over a multipath fading channel, thereis frequency selectivity over the entire bandwidth, but eachcarrier experiences a unique flat fade (typical of OFDMtransmission), At the receiver side, the received signal (aftertransmission over a frequency selective fading channel), y(n),is converted back to frequency domain (by applying a Npoint FFT) to create a discrete N point sequence Y ( n . ) ,ncorresponding to each sub-carrier. The demodulated symbolstream is given by:

N-1~ ( m )C y(n)e-~* -+ w(m) (3 )

where W(m)corresponds to the FFT of the samples ofw (n) , which is the Additive White Gaussian Noise (AWGN)introduced in the channel.III. OFDM IN UAV C O M M U N I C A T I O NH A N N E L

The propagation effects that must be accounted for in theUAV channel model are: time delay spread, Doppler spread,power, frequency of operation, free space loss, fading rate,multipath diffraction and shadowing, as well as general back-ground noise. Often caused by multipath conditions, fadingcan degrade the Bit-Error-Rate (BER) performance.However, when OEDM is being applied in UAV communi-cations, Doppler shift becomes a crucial component. Dopplershift is the frequency shift experienced by the radio signalwhen either the transmitter or the receiver is in motion, and

n=O

Doppler spread is a measure of the spectral broadening causedby the t h e rate of change of mobile radio channel. TheDoppler spread in the frequency domain is closely relatedto the rate of change of the envelope of the received signal.Since UAVs are flying at high speed, a large Doppler spreadis observed.More importantly, one of themain disadvantages of OFDMsystem is its susceptibility to small differences in frequencyat the transmitter an d the receiver (normally referred to asfrequency offset). When the Doppler spread is large, orthog-onality among OFDM ubcarriers is lost, i.e.,

lTo s ( 2 ~ ( m . 4 f + s f , ) t + # ~ ) c o s ( 2 ~ ( ~ . ~ ~ ~ ~ ~0(4)where 6fm is the Doppler frequency shift at mth carrier.As a direct result of loss of orthogonality, Inter Carrier In-terference (ICI) is introduced and 3ER erformance degrades.Now lets analyze the effects of Doppler spread in mobi!echannels on OFDM systems.We us e a maximum UAV speed of 250 miles per hour herein the analysis. Since both ends of the communication system

are moving, the relative speed between two UAVs could be ashigh as 50 0 miles per hour. The worst case maximum Dopplershift corresponding to the operation at 5 GH z (IEEE 02.1 lastandard) is [lO][ll]

The coherence time of the channel corresponds to theDoppler shift, is shown as [10][11]

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Without loss of generality, we consider the Ot h carrier andits carr ied data symbol. In AWGN channel, it is easy toshow that the signal to interference and noise ratio (SINR)is (assuming BPSK modulation)S I N E =

T, =$"-6 . ~ .z= 0.114ms (6)This means an update rate of about 8.75KHz is requiredfor channel estimation and equalization. Consequently, thepacket size of OFDM ystem needs to be adjusted (decreased)accordingly to the high update rate.When IC1 is observed at the receiver side, the receivedOFDM signal is given by [9]

(11)b

where E[.] s the mathematical expectation. Hence, the BER

0 1

O C B -

008 -

0 0 7 -

006 -fgam-

of OFDM system with IC1 corresponds toy(r1) = z'(n)ej? + W(.) (7)

-up symbols (and 4 pilot tones). To model realistic UAV wirelessenvironments, the Rayleigh fading channel employed in oursimulation demonstrates frequency selectivity over the entirebandwidth, BW, but flat fading over each of the N carriers.Specifically, we assume a channel model with coherence

---. bandwidth, (Af)c, characterized by

(Af),/BW = 0.25 (15)

where d(n) s the received OFDM signal transmitted overoffset. Given fm s the frequency offset an d Ts is the OFDMsymbol duration,the fading channel without ICI, E is the normalized frequency , BER = Q[I (12)

3 -t ls(1)12

7.03 As a result, the ai's in the 52 carriers are correlatedaccording to(16)1

where Ri; enotes the correlation between the if h canierlo l5 25 35 45 5o and the jfb arrier, and (fi - f,) is the frequency separation

between these two carriers. Generation of correlated fades, forFig. 2. IC1 Ccefficienls for IEEE 802.11a OFDM System purposes of simulation, has been discussed in [12].

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Figure 3illustrates the simulation results in AWGN channel.Figure 4hows the simulation results in frequency selectivefading channel. In Figures 3and 4, r axis represents S N R ndy axis shows BER. The curve marked with circles representsthe BER performance in UAV channel and the curve markedwith s ta rs represents the BER in wireless indoor channel(where Doppler spread and IC1 is minimum). From Figures3 and 4, it is obvious that the performance degradationintroduced by high mobility in UAV communication channel isnegligible in AWGN channel and very small in fading channel(about 2dB loss observed at BE R = 3

BEA p r k of OFDMw i h IC1n AWGN4 m G I 1--c w tonout IC1 :

E o -

10- :1

O t 2 3 4 5 6 7 8 9SN A10

fig. 3.Channel BER Performance of E E E 802.tla OFDM System in AWGN

I5 10 15 XI 25 35 (0SNR

Fig.4.Channel BER Performance of IEEE 802.11a OFDM System in UAV Fading

Figure 5 illustrates the effect of the flying speed of UAVson BER performance degradation of OFDM transmission in

UAV fading channels. In Figure 5 , 5 axis shows relative speedbetween OFDM transmitter and receiver in miles per hour, andy axis is BER. The curve marked with circles represents theBE R performance at S N R = 30dB and the curve markedwith stars represents the BER at SN R = 4 0 d B . It is obviousthat when the relative speed grows, the BER performance ofOFDM system degrades. Nevertheless, when SNR is low, theBER performance degradation is graceful, while SNR s high,the BER performance degrades drastically. This differenceis due to the fact that IC1 to noise power ratio increaseswith SNR: when SNR is high, IC1 dominates the noise andperformance degradation due to IC1 s more severe. As can beseen in Figure 5 , the performance degradation at reasonableSNR and reasonable flying speed of UAV is well tolerated.

a

Fig. 5. BER Performance of OFDM System at Different SpeedTo summarize, when OFDM transmission is introducedto UAV wireless communications, only a slight performancedegradation is observed due to larger Doppler spread and InterCarrier Interference. Hence, the application of current OEDMsystem into next generation swann UAV communications is afeasible choice.

V. CONCLUSIONSIn this paper, we analyzed the application o f OFDM nt oUAV wireless comm unications. Specifically, Doppler spreadand coherence time have been calculated to determine theproper OFDM system parameters. Next, the Inter CarrierInterference (ICI) an d its power have been derived. ThenBE R performance of OFDM system in UAV wireless com-

munications with large Doppler spread and large IC1 hasbeen analyzed. Simulation results show that the performancedegradation of OFDM ystem due to large Doppler spread andIC1 is we11 tolerated and OFDM is a feasible choice for nextgeneration swarm UAV communications.

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ACKNOWLEDGMENTThis work was supported by DoD through Alion Sci-ence Technology an d Augusta Systems, Inc. Contract NO :SUB1166497lU3 and performed at the Center for Researchon Advanced Control of Autonomous Systems and Manufac-turing, L .C. N. College of Engineering WVU Tech.The authors would like to thank Yue Hu and Travis Mc-Cartney for their useful comments.

REFERENCES[ l ] Hanison Donnelly, swarm UAVs, Milirur?,Aerospace Technology, http://w w .m il my-aerospace-1echnology.~omlprintanicle.~fm?DoclD=686[2] E V. Ne e and R. h a d , OFDM o r Wireless Multimedia Cammunica-rims. Anech House, Boston, 2000[3] B. Le Flock. M. lard, and C. Berrou, "Coded onhogonal frequencydivision multiplex", Proceedings o zhe IEEE, Vol. 83 , no. 6, pp 982-996, Iune1995.141 IEEESfd 802 .11~-1999 ,Draft supplement io standard for telecommuni-cations and information exchange between systems - LAN/MSN specificrequirements- Part 11: wireless MAC and PHY pecifications: High speedphysical layer in the 5 GHz band," P802.1 la/D6.0, May 1999.[SI / E P802.11g-2003 - TASK GROUP G - Project IEEE 802.11g Sun-dard for Higher Raw (20 t Mbps) Extensions in the 2.4GHz Band[6] 573, "Broadband Radio Access Networks (BRAN); HIPERLAN Type 2Technical Specification Part 1 - Physical Layer," DTSIBRANO30003-1.

Oct. 1999.[7] IEEE P802.16-2004, standard for local and metropolitan area networks[8] Hassan Yaghoobi. Scalable OFDM Physical Layer in IEEE 802.16Wireless MAN, Inre/ Technology Journal,Volume 8, ssue 3, pp. 201-212.2004191 M. nandpara, E. a , . Golab, R.Samanta, H . Wang. T. S.Rappaport,Inter-Carrier Intqference Cancellation for OFDM Systems[IO] I. G.ha!&D i g i d Communicduns , McGraw-Hill, New York, 1995[I I ] T. S.Rappaport. Wireless Communications: Principles and Practice.Predtice Hall, New Jersey. 1996[I21 B.Natarajan, C.R. assar and V. Chandrasekhar,"Generation of Corre-lated Rayleigh Fading envelops for spread spec" applications", IEEECummunicntion Letters, vol. 4. na.1. Jan, 2000, pp. 9-1 1.

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